Some time in the next decade, cars powered purely by petrol are likely to become passe for the fashion-conscious driver.

Toyota has committed to building a hybrid version of every car in its range by 2020, while Nissan says 10 per cent of its cars are likely to use no petrol at all by the beginning of the next decade.

drive cover

But buying a car in the post-fossil fuel era is not going to be as crystal clear as the average car buyer would hope. For a start, there are three types of hybrids available already, with another two set to launch in the next year or two. Some government departments are already leasing electric vehicles in Australia, while overseas there are trial lease programs for fuel-cell vehicles that run on hydrogen.

Then there are ethanol-blend, natural gas and biodiesel vehicles that work on the same principle as the petrol engine, with a different fuel source.

Dig a little deeper and there are more choices to be made: lithium-ion or nickel-metal hydride batteries; corn, wheat, sugar or inedible plant waste-based ethanol; compressed natural gas or LPG; and finally, coal, wind or solar-powered electricity.

It all makes getting an accurate reading on the size of your car's carbon footprint a mind-boggling prospect. In the past a car's fuel use was the accepted rule of thumb for its environmental impact - the more fuel you used, the more CO2 and pollution you emitted - but now there are many more variables.

Related Content

Take hybrids, for example. Critics argue more CO2 is produced in the building of a hybrid than in a conventional car; additional battery components have to be mined and shipped overseas during the manufacturing process.

The locally built Toyota Hybrid Camry is a good example. The standard car's petrol engine is built in Australia yet the hybrid's components are shipped in.

Electric vehicles have similar whole-of-life emissions drawbacks. Analysis has found they are significantly worse for the environment than hybrid petrol-electric vehicles if they are powered by coal-fired power stations.

Drive has compiled a beginner's guide to the alternative-fuel highway, looking at the technology behind the new breed of electric and hybrid cars - and the pros and cons of each.

Micro-hybrids

Mercedes-Benz markets a "micro hybrid drive" version of its quirky Smart Fortwo two-seater but the car isn't a hybrid in the true sense of the word.

It has no electric motor supplementing its petrol engine. Instead, it uses a belt-driven generator that feeds charge into the car's electrical system and also acts as a starter motor.

The generator allows the car to shut off its engine in stop-start traffic to conserve fuel. When the car pulls up at a set of lights, the engine switches off until the accelerator is pressed and the generator kicks the internal combustion engine back into life.

The stop-start system has been used on hybrids for some years now but other car makers are adopting the technology.

Luxury brands such as BMW also use a smart alternator system that only recharges the battery when the car is braking or coasting. It reduces the load on the internal combustion engine, therefore cutting fuel use.

Other similar systems use a flywheel as a generator, capturing mechanical rather than electrical energy during the braking process. The flywheel acts like a giant elastic band that winds up under braking and unravels when you push the accelerator. The idea is not new; Leonardo da Vinci drew sketches of a rubber band-powered car centuries ago.

Some formula one teams experimented with flywheel-based kinetic energy recovery systems (KERS) last season, while Porsche is using a similar type of hybrid system in a GT3 race car with the intention of transferring the technology to a road car in the near future. The car has a conventional six-cylinder petrol engine powering the rear wheels, combined with two 60kW electric motors over the front axle sending power to the front wheels.

When the car brakes, the two motors turn into generators, feeding mechanical energy into a large flywheel, where it is stored until needed. That energy can then be tapped to provide an extra 120kW of power for a six-to-eight second burst. It only takes one big stop from top speed to recharge the system in full.

Hybrid - parallel or 'mild'

A parallel hybrid uses a small electric motor to supplement the power of the petrol engine when required. The motor is not powerful enough to propel the car on its own.

The fuel savings achieved by the electric motor are complemented by technologies such as stop-start systems and regenerative braking, which captures kinetic energy that is usually lost in friction and heat via the brake pads and discs.

On a conventional car, the brake pads clamp on to the brake discs to stop the wheels from turning, bringing the car to a halt. But a regenerative braking system uses engine braking to slow the vehicle. When the car begins to decelerate, the electric motor in effect goes into reverse and becomes a generator, collecting electrical energy and storing it in the car's battery for later use. The system takes some of the load off the petrol engine by powering ancillary systems such as the heating and airconditioning.

The combined fuel savings allow a mild hybrid to deliver the performance of a larger petrol engine, with the fuel savings of a smaller engine.

Honda favours the parallel hybrid system and uses it in the current Honda Civic hybrid. The company says it is a lower-cost, more widely accessible alternative to full hybrids such as the Prius. The technology also requires less battery power, which means the battery pack weighs less and doesn't take up as much space.

But the Civic hybrid isn't as green as it appears, according to the federal government's Green Vehicle Guide, which rates the petrol version of the Civic as better for the environment than the hybrid version because it emits fewer hydrocarbons. To be fair, that rating has less to do with the hybrid system and more to do with the fact that the standard car's 1.8-litre petrol engine is cleaner than the 1.3-litre petrol unit used in the hybrid.

The Honda hybrid system, called integrated motor assist, will also appear later this year in the new Insight and next year in the CR-Z sports car. Honda claims the 1.5-litre four-cylinder in the CR-Z gives it identical torque - or pulling power - to the larger 1.8-litre engine in the Civic, while using almost a quarter less fuel.

But the technology is still expensive compared with recent turbocharged petrol engines that can deliver similar results. The 1.4-litre turbocharged and supercharged engine in the latest Volkswagen Golf is more powerful than the old 2.0-litre model and almost one second quicker in the 0-100km/h sprint but uses 22 per cent less fuel.

And the 1.2-litre turbocharged engine in the VW Polo has almost identical power and torque figures to the CR-Z, while using only half a litre of fuel more per 100 kilometres.

Other manufacturers have followed Honda's lead by producing mild hybrids. Mercedes-Benz and BMW both have mild hybrid systems for their top-end limousines in the US, while other domestic US vehicles are also mild hybrids.

Ferrari is also looking at mild hybrid versions of its grand tourers.

Full hybrid

Parallel-series hybrids, or "full hybrids", were popularised by Toyota's Prius.

They use bigger electric motors that can power a vehicle on their own at low speed for a short distance (about one kilometre).

When the car is cruising, both the engine and the electric motor drive the wheels and any surplus engine power is used to recharge the battery. The Prius also has regenerative braking and stop-start technology.

The full hybrid system translates to better fuel efficiency than a parallel hybrid, particularly in bumper-to-bumper city traffic where the engine can run on electric power only. Mild hybrids are at their most efficient on the highway, which is arguably less relevant to the average motorist. The trade-off with a full hybrid is a higher price tag and bigger batteries that take up valuable boot space.

The cheapest Toyota Hybrid Camry you can buy in Australia is $36,990 (plus on-road and dealer costs), a $6200 premium over the cheapest petrol version. Toyota argues the hybrid model has more equipment to justify its price tag but it nevertheless makes the car less accessible.

The Prius is $39,990 (plus costs) but is smaller than a Camry.

The Volkswagen group has followed Toyota's lead and developed a similar hybrid system, although the company still appears ambivalent about its merits. A hybrid version of the VW Touareg will not make it to Australia because VW says its diesel-powered Touareg is more fuel-efficient.

VW-owned Porsche, on the other hand, has become the second luxury brand - behind Toyota's luxury arm, Lexus - to launch a hybrid SUV in Australia.

Lexus and Porsche have been criticised in some quarters because their hybrids are performance-focused, delivering better acceleration than petrol versions of the same car. Critics argue the potential for greater fuel savings has been sacrificed for performance.

Series hybrid or range-extender electric

In a series hybrid car, the petrol engine never drives the wheels. Instead, it acts as a generator, replenishing the car's battery, which then powers the motor.

General Motors is pioneering full-scale production of the series hybrid with its much-vaunted Volt, although you'll never hear the company's marketing boffins use the "h" word.

According to GM, the Volt is a plug-in electric vehicle with a range extender. The term reflects GM's belief that the car's internal combustion engine will only be used on weekend getaways and holidays.

The company says its research indicates that the car's claimed electric-only range of 60 kilometres will be enough for most motorists' average daily commute. But on its official website, the company admits its range estimate will depend on "the weather, electrical features in use and how you drive".

Exponents of range-extender models - Jaguar said recently it thought the system was superior to Toyota's hybrid system, while Lotus is developing similar technology - say they are more efficient than full hybrids because the petrol engine is always running at a constant, fuel-efficient speed and can be tuned accordingly. Toyota, predictably, disagrees.

GM also says the range-extender is preferable because it overcomes range anxiety, where people are scared of being stranded if they have to take an unexpected detour. They claim the petrol engine will provide another 480 kilometres of range.

But Nissan, which has recently launched its electric Leaf hatch in Japan, says the GM solution is ultimately a stop-gap because it has to carry around - and the buyer has to pay for - a 1.4-litre, four-cylinder petrol engine that is idle for large periods of time.

Ultimately, cost is one of the biggest barriers to vehicles such as the Volt and Leaf. They are incredibly expensive for what they provide.

In the US, the Volt will sell for $US41,000 ($45,900), although a $US7500 tax saving will bring the total outlay to $US33,500. That is still big money for what is essentially a small car, especially when you can get a Chevrolet Camaro for less than $US23,000.

Questions have also been raised about the new-generation lithium-ion batteries in the Volt, which are largely untried in an automotive application. No one doubts that they are superior to the older, bulkier and heavier nickel-metal hydride batteries used in the Toyota Prius but Toyota says it stuck with the old battery technology for its new model Prius last year because they were proven and reliable. GM says lithium-ion delivers better power and energy density, as well as a longer cycle life. To dispel doubts, GM has an eight-year, 160,000-kilometre warranty on the batteries.

Toyota is conducting trials on a plug-in version of the Prius that will have an electric-only range of about 20 kilometres before switching to hybrid operation.

Electric vehicles

Electric vehicles are seen by many as the most sensible mid-term prospect for a zero-emissions future, at least for those who don't need to travel long distances regularly.

With no CO2 emissions from the tailpipe, they have the potential to substantially improve the quality of air in big cities.

But there are still big hurdles to acceptance of the technology, including cost, infrastructure and driving range.

Australia's first electric cars have already arrived from overseas - a batch of 40 Mitsubishi i-MiEVs destined for government and business customers and a high-end sports car from small US outfit Tesla. They will be followed in 2012 by the Nissan Leaf and Chevrolet Volt, while a Chinese brand is expected to bring an electric car here for as little as $25,000.

In recognition of the new technology, the Department of Transport is planning to change the labels cars are required to carry in the showroom.

The new labels will look more like those you'd find on a washing machine than a car, with energy usage ratings rather than customary fuel-use numbers.

But the labels have already drawn fire from critics for their assessment of a car's environmental impact.

Electric vehicles will get a zero on their label for both fuel use and CO2 emissions ? a move critics say doesn't tell the whole life-cycle story of an electric car.

The label has a disclaimer that reads: "CO2 emissions can also be generated at the power source when vehicles are recharged, unless 100 per cent renewable energy is used."

But it makes no mention of claims that many electric and hybrid vehicles generate more CO2 than conventional vehicles in the production and shipping process. The government's Green Vehicle Guide site will have a calculator that allows motorists to estimate their CO2 emissions based on the energy source they use.

It's likely that a Mitsubishi i-MiEV driven in hydroelectric-friendly Tasmania could be a substantially greener alternative than one driven in Victoria or NSW, which rely more on brown coal for electricity production. However, cars recharged from coal-fired power stations in off-peak times are, theoretically, using energy that would otherwise go to waste.

But the question of environmental impact will be just one headache facing electric car buyers.

The i-MiEV is being leased rather than sold to Australian customers and private buyers are unlikely to get a look-in because Mitsubishi courts high-profile clients that can promote the car to the wider community. The cost of a three-year lease is an eye-popping $62,640 - and the owner has to hand the car back to the company at the end of that period.

Electric car advocates argue the price of electric vehicles will plummet once they begin selling in higher numbers.

But where will that leave the early adopters, who are likely to take a huge hit when they want to upgrade? Overseas, vehicle resale experts claim five-year-old EVs could be worth as little as 10 per cent of their original value.

Fuel-cell vehicles

Fuel-cell vehicles are arguably the cleanest form of zero-emissions transport because, rather than drawing their energy from conventional power sources, they create electricity on board.

Electricity is generated via membranes that mix hydrogen - the universe's most abundant element - with oxygen, emitting only water vapour from the tailpipe.

But if you thought electric vehicles were expensive, then the price tag on your average fuel-cell vehicle will blow you away.

The world's first "production" fuel-cell vehicle, Honda's Clarity, is believed to be worth an estimated $330,000, although only 200 will be made during the next couple of years and all will be leased, rather than sold.

But Honda believes it can have a commercially viable fuel-cell vehicle on the market by 2018 and rival Toyota says it could have a fuel cell that is price-competitive with electric vehicles in just five years.

Toyota has reportedly drastically reduced the unit cost of producing a fuel-cell vehicle from $1 million to $100,000 in the past few years and believes it can bring one to market for about $50,000 by 2015.

Apart from the environmental advantages, fuel-cell vehicles also overcome two key problems of plug-in electric vehicles. They are expected to deliver a range similar to petrol cars and can be refuelled in a couple of minutes, compared with several hours for a plug-in.

Infrastructure remains a major hurdle, though. For fuel-cell vehicles to work, there needs to be an extensive network of hydrogen fuelling stations to support them.

Sourcing the hydrogen can also be problematic. Extracting it from water requires more energy than you get out of it.

Another hurdle is the US government, which has gone cold on hydrogen and directed billions of dollars into electric vehicles at the expensive of fuel cells.

Honda says the move is short-sighted and that electric vehicles are a quick fix, while fuel cells are the "ultimate solution".

With so much indecision and uncertainty surrounding the ultimate way forward, it's not time to throw away the keys to your old gas guzzler just yet.